There are a variety of methods known for generally determining the health of gas turbine engines, particularly aircraft engines. General engine condition, as well as an indication of engine life expectancy and need for overhaul, is provided by trending systems which utilize engine parameters such as various temperatures, pressures and control parameters associated therewith to determine current engine operating condition and impute engine health. However, such systems do not recognize isolated events within the engine which can be indicative of severe engine distress or impending engine component failure from a particular causal event.
Certain engine conditions can be determined visually, such as through borescopes, without tearing down the engine. As examples, severe blade erosion (high temperature corrosion), loss of abradable seal segments, or excessive rubbing can frequently be detected by borescope inspection methods. Additonally, periodic teardown of an engine allows inspection of far more components, much more reliably. Because engine teardown is such a complex and expensive proposition, various schemes are employed to determine when engine teardown should be performed. Tearing engines down too frequently is, of course, an extreme waste of time and money. Failure to tear down an engine when it may have problems could result in engines malfunctioning while in use. Because of the complexity and expense involved, any improvement in diagnostic methodology is of great value.
In the past decade, monitoring of the electrical characteristics of gas flowing through a jet engine has been studied as a possible indication of engine deterioration. Apparatus disclosed in U.S. Pat. No. 3,775,763 utilizes an electrostatic probe positioned in the exhaust of the jet engine, such as through the tail pipe wall. Abnormal conditions were thought to be coupled with small particles striking the probe and causing spikes of ion current of a relatively large magnitude. Subsequently, as reported by Couch, R. P.: "Detecting Abnormal Turbine Engine Deterioration Using Electrostatic Methods", Journal of Aircraft, Vol. 15, October 1978, pp 692-695, it was theorized that the signals did not result from individual particles of metal hitting the probe, but rather that the signals were indicative of Trichel pulses (a form of repetitive corona discharge) created by high potential pockets of excess charge. A probe set including circular insulated segments within the gas turbine engine tail pipe and a triangle of wire extending through the tail pipe exhaust gas path were developed. With these probes, a normalized count of large signals (probe current, or voltage developed across an impedance, in excess of a threshold magnitude) over a period of time definitely correlated with impending engine component malfunctions or severe deterioration. As reported in the aforementioned article, however, the use of normalized counts of large magnitude signals from the ring and grid probe was thought to provide reliable prediction of only two out of three gas-path failures, at best, and distinction between possible causes thereof was highly experimental, as described below.
In a commonly owned, copending U.S. patent application entitled "Waveform Discriminated Electrostatic Engine Diagnostics", Ser. No. 454,124, filed contemporaneously herewith by Zwicke et al, characteristics of the waveshape of signals developed on an electrostatic probe disposed for response to electrostatic charge in the gas stream of a gas turbine engine are utilized to discriminate waveshapes correlated to particular engine events or conditions, or classes thereof, from other waveshapes not having those characteristics or having other characteristics, which are thereby identified as different, whether correlated with other events and conditions or not.
In the waveshape-discriminated diagnostic system disclosed in the aforementioned application, it is shown that a large number of electrostatic events in the gas stream of a gas turbine engine which are indicative of abnormal engine component wear, typically occur wholly within on the order of 10 to 60 milliseconds. It is further shown, therein, that engine events which cause the electrostatic signals being monitored occur randomly, and relatively infrequently. Thus, if an engine is not experiencing abnormal wear of a component, only two or three events may occur in the period of an hour. When abnormal wear of an engine component does result in electrostatic signals, twenty or thirty signals may occur in a relatively shorter period of time. In order to analyze the waveshapes, it is necessary to sample them at a sufficiently frequent interval so as to be able to reproduce them, digitally, with sufficient accuracy so as to enable classificatio,n thereof. This means that sampling of the data may occur at on the order of one thousand times per second. On the other hand, only tens of events will occur in a million samplings at that rate.
The waveshapes of interest are those in which the amplitude variations of the electrostatic charge being monitored, and therefore the amplitude variations of the resulting electrostatic signals, are of a magnitude sufficiently in excess of noise so as to be indicative of a real event, and so as to be capable of correlation through waveshape analysis (not masked by the noise). The waveshapes are, at times, indicative of the end of an event which commenced at an earlier time. Some of the waveshape characteristics that are useful in classifying the waveshapes with respect to corresponding, correlated engine events occur prior to the magnitude or amplitude of the waveshape exceeding the threshold (some factor above background noise).
In our aforementioned application, abnormal wear is recognized when ten times a normal number of events occur. Thus in a given flight or cycle of operation on the test stand, on the order of two rubs or erosions or the like may occur; if twenty events occur, this is taken as being indicative of abnormal wear worthy of being analyzed to determine the causation thereof, as set forth in a commonly owned, copending U.S. patent application entitled "Expanded Classification Sample In Electrostatic Engine Diagnostics", Ser. No. 453,967, filed contemporaneously herewith by Couch. In the event that less than an abnormal number of events occur within a given interval, all of the records of waveshapes which have been made are useless, and are thrown out.